According to the invention there is provided a method and an apparatus for plasma etching a workpiece.
The etching of silicon wafers is an important process in the industrial manufacture of electronic components. It is preferable for the etching profile to be uniform across the entire surface of the wafer. Ideally, such a uniform and high quality etch should be achieved to within 3 mm of the wafer edge to prevent unnecessary waste of the wafer material.
Current methods used to etch a silicon wafer include plasma etching using a cyclical technique commonly known as the “Bosch process”. In this technique, alternate deposition steps and etch steps are performed cyclically. The Bosch process is well-known in the art and is described, for example, in U.S. Pat. No. 5,501,893. However, due to a number of discontinuities towards the wafer edge, such as changes in gas flow, temperature and plasma density, it can be difficult to maintain the uniformity of this plasma etch process. Methods to control the plasma uniformity and minimise these edge effects include using electrostatic chucks (ESCs), gas flow management, and platen assemblies which are larger than the wafer. Focus rings, which may be ceramic or silicon annular rings, may also be used to control the plasma uniformity. However, the plasma uniformity may deviate during the plasma etch process as process conditions, such as RF power and chamber pressure, fluctuate. This can lead to asymmetries in the plasma etch process, in particular towards the wafer edge where the incident angle of ion bombardment can be more difficult to control and reproduce. An SEM image of such an asymmetry at the base of a via etched in silicon is shown in FIG. 1. Such an asymmetry would lead to defective die.
In the plasma etching of silicon wafers, a negative polarisation of the wafer surface is typically achieved such that positive ions from the plasma are attracted to the surface of the wafer. The region in the plasma from where the positive ions have been extracted is known as the plasma sheath. The positive ions bombard the wafer surface in a direction generally perpendicular from the plasma sheath. Therefore distortions in the thickness or tilt of the plasma sheath will lead to non-uniformities in the etched surface. The tilt of the plasma sheath corresponds to the incident angle at which ions bombard the surface of the substrate. The tilt angle is measured between the incident path of the ions and the normal to the wafer surface. The incident angle of ion bombardment at the edge of the wafer may be tilted with respect to the incident angle of ion bombardment at the centre of the wafer. The thickness and tilt of the plasma sheath is dependent upon plasma density, local potential and topology of the wafer. These parameters vary throughout the process chamber and lead to variation in plasma sheath thickness across the surface of the wafer. This plasma sheath distortion is most pronounced at the wafer edge where material and mechanical discontinuities are present. For example, there is often a local voltage variation on the surface of the substrate towards the wafer edge. This generates a distortion in the plasma sheath thickness and can lead to an asymmetric etch or an etch with a tapered profile. FIG. 2 illustrates a cross-section of the edge of a substrate 20 placed on a RF driven electrostatic chuck (ESC) 22 in the presence of a plasma, where the plasma sheath 34 has an outward tilt at the edge of the substrate 20. The substrate 20 is positioned on an ESC 22 and exposed to a plasma 32 generated using a suitable etchant source gas. In this example, the plasma sheath 34 has an outward tilt at the edge of the substrate 20 such that the incident angle of ions bombarding the substrate surface has a radially outward component.
The plasma etching process may involve a main etch followed by an over etch, which is typically commenced once an end-point condition has been detected. The main etch cycle is also known as a bulk etch. The main etch may utilise the cyclic deposition and etch Bosch process. The main etch can be used to form trenches or vias, and the over etch cycle is performed to ensure that the etch profile is uniform and complete across the wafer surface, and to remove any other residual material from the vias which were deposited from the main etch process. An initial, “breakthrough” etch step may also be performed. In general, different etch recipes and process parameters are used during the breakthrough etch, Bosch main etch and over etch sequences.
The tilt of the plasma sheath can be optimised during the over etch cycle such that the over etch is aligned as far as possible with the main etch. In general, this involves adjusting the process parameters to minimise the plasma distortions between the main etch cycle and the over etch cycle, which can often be complicated. Despite the process parameter optimisations, there are usually small variations in the plasma sheath between the main etch and the over etch. However, even small variations in the tilt between the main etch and the over etch can lead to a tapered or asymmetrical profile for the over etch. Such asymmetries will become more pronounced at the base of features such as vias with a high aspect ratio. These asymmetries may lead to defective die.